During a conversation on IRC, I started to think about our network peers
that have IPv6, but don't send us any of those routes.

The disjoint in IPv4 peers vs IPv6 peers is extremely noticeable with BT
(http://bgp.he.net/AS2856), but is common with almost all network
entities.

BGP Peers Observed (v4): 200
BGP Peers Observed (v6): 16

As best I can tell, there are two basic reasons why I would receive
IPv4 routes and not IPv6 routes from a peer.

First, is that peer does not announce any IPv6 routes to anyone. Maybe
they don't have an allocation, maybe they have one but aren't using it.
Who knows. Personally, I don't see much of a problem with that; even
though that network really should get working on IPv6 for themselves.

The second reason is they only announce IPv6 to (at least) one transit
provider, but not to their network peers. Maybe it's just to keep
others from squatting on "unused space", maybe it's not really used, so
they don't put any effort in. More commonly though, I believe it is
from a (general) lack of effort put into IPv6. Looking at my own
IXP connected routers, I see many ASes that are announcing IPv4 to the
route server but not announcing their IPv6 networks to the route server.

Why is that? Did they get a great price on IPv6 traffic from their
transit providers? Are they not as interested in traffic going over the
Exchanges? Simply forgot? Is their backbone not prepared for IPv6 yet,
and this is something they plan on fixing in the future?

While I don't think IPv6 is the end-all-be-all magical fix to all of our
networking woes, I think the ship has sailed and we all need to get up
to speed on IPv6. Network interconnections is one of the lowest levels
of infrastructure, and needs to be done extremely early on in an IPv6
deployment to ensure high quality networking for users of that protocol.